1. Field of the Invention
The present invention relates to a bicycle front derailleur for shifting a chain from one sprocket to another of a multiple chainwheel to thereby perform an intended speed change.
2. Description of the Prior Art
In general, a front derailleur associated with a multiple chainwheel (front gear) is mounted on a bicycle usually in combination with a rear derailleur associated with a multiple freewheel (rear gear), so that the rider can select a greater number of speeds than possible with the rear derailleur alone. When the chain engages a larger sprocket of the chainwheel and/or a smaller sprocket of the freewheel, a larger gear ratio will result to enable higher speed running. Conversely, when the chain engages a smaller sprocket of the chainwheel and/or a larger sprocket of the freewheel, the gear ratio will become smaller to cause the bicycle to run at a lower speed with a higher torque.
The bicycle front derailleur includes a chain guide which has inner and outer guide plates respectively located on both sides of the chain extending between the front and rear gears. The front derailleur is operated to translate the chain guide laterally of the bicycle, namely axially of the front gear, thereby pressing the chain at a position thereof advancing into engagement with the front gear. Most commonly used for moving the chain guide in this way is a parallelogrammic linkage mechanism, as disclosed for example in FIG. 6 of U.S. Pat. No. 4,674,995. When the rider manipulates a shift lever on the bicycle frame, a control cable connected to the shift lever is pulled to deform the linkage mechanism, thereby translating the chain guide laterally of the bicycle. This causes the inner or outer guide plate of the chain guide to push the chain laterally, whereby the chain in engagement with one sprocket of the chainwheel is shifted to another sprocket of the chainwheel to change the bicycle running speed.
The parallelogrammic linkage mechanism usually incorporates a return spring which functions to deform the linkage mechanism in a returning direction. When the rider pivots the shift lever to pull the control cable, the linkage mechanism is deformed against the return spring. On the other hand, when the rider pivots the shift lever to pay out the cable, the linkage mechanism is deformed in the returning direction by the elastic force of the return spring. In this way, the reciprocal pivotal movement of the shift lever causes the chain guide to translate reciprocally relative to the front gear, thereby enabling the speed change.
The multiple chainwheel (front gear), which is mounted to a pedal crank assembly, consists of diametrically different sprockets which include a largest sprocket (top gear) located laterally farthest from the bicycle frame, and a smallest sprocket (low gear) located laterally closest to the frame. Bicycle front derailleurs incorporating a parallelogrammic linkage mechanism are classified into two types, namely "top normal type" and "low normal type", depending on the direction in which the return spring of the linkage mechanism functions to return the chain guide.
With a front derailleur of the top normal type, the parallelogrammic linkage mechanism is deformed by the urging force of the return spring to return the chain guide to a position corresponding to the largest sprocket (top gear) when the control cable is maximally paid out. Conversely, with a front derailleur of the low normal type, the parallelogrammic linkage mechanism is deformed by the biasing force of the return spring to return the chain guide to a position corresponding to the smallest sprocket (low gear) when the control cable is maximally paid out.
The front derailleur disclosed in U.S. Pat. No. 4,674,995 above belongs to the low normal type. The present invention, on the other hand, concerns bicycle front derailleurs of the top normal type.
Before describing the present invention, reference is now made to FIG. 3 which shows a conventional bicycle front derailleur of the top normal type.
As shown in FIG. 3, the conventional front derailleur comprises a parallelogrammic linkage mechanism generally represented by reference numeral 10. The linkage mechanism comprises four members, i.e., a base member 2 fixed to the seat tube 1 of the bicycle frame as by a clamp band, an inner link 3 pivoted at its one end (upper end) 3a to the base member 2 by a pin 5, an outer link 4 pivoted at its one end (upper end) 4a to the base member by another pin 6, and a movable member 9 pivoted to the respective other ends (lower ends) of the inner and outer links by further pins 7, 8.
The four pins 5, 6, 7, 8 pivotally connecting the four members 2, 3, 4, 9 together are located at the four corners of a parallelogram. Thus, when the linkage mechanism 10 deforms, the movable member 9 translates laterally toward and away from the bicycle frame.
The movable member 9 is integrally formed with a chain guide 11 which includes an inner guide plate 11a and an outer guide plate 11b. Both guide plates are generally parallel to each other and spaced apart by a distance permitting a chain C to pass therebetween. The chain guide 11 is located above a chainwheel S close thereabove. For the purposes of simplicity, the chainwheel S is shown to have only two sprockets which include a smaller sprocket Sa and a larger sprocket Sb.
The linkage mechanism 10 is always biased by a return spring 15 so as to move the chain guide 11 laterally away from the bicycle frame, i.e., toward the larger sprocket Sb. The outer link 4 has an intermediate portion provided with a connector 14 for connection to one end of a control cable 13.
The control cable 13, which is connected at the other remote end to the shift lever (not shown), extends downward along the down tube (not shown) of the bicycle frame, and then upward generally along the seat tube 1. Further, the cable is directed laterally away from the seat tube 1 by a cable guide 12 fixed to the base member 2 before being connected to the connector 14.
The shift lever is reciprocally pivotable to wind up or pay out the cable 13. Thus, the cable may be moved lengthwise thereof to operate the front derailleur.
When the rider manipulates the shift lever to pull the cable 13 in the direction of arrow P in FIG. 3, the link mechanism 10 deforms against the elastic force of the spring 15 to translate the chain guide 11 in the direction of arrow p. As a result, the outer guide plate 11b of the chain guide 11 presses the chain C engaging the larger sprocket Sb into shifted engagement with the smaller sprocket Sa.
Conversely, when the shift lever is reversely operated to pay out the cable 13 in the direction of arrow Q, the linkage mechanism 10 restoratively deforms under the elastic force of the spring 15 while the degree of restoration is regulated by the amount of the cable pay-out. As a result, the chain guide 11 translates in the direction of arrow q, so that the chain C is pushed by the inner guide plate 11a again into engagement with the larger sprocket Sb.
As apparent from FIG. 3, the conventional front derailleur of the top normal type necessitates an arrangement in which the control cable 13 must pass at least two directional change points or bent points. Specifically, a first bent point is located at the frame bottom lug where the seat tube 1 is connected to the down tube (not shown), whereas a second bent point is positioned at the cable guide 12. In fact, such a cable arrangement gives rise to the following drawbacks.
First, since the control cable 13 is bent twice to a great degree at the bottom lug of the bicycle frame and the cable guide 12, the cable is subjected to a large frictional resistance which greatly impairs the operability of the shift lever. Further, the large frictional resistance causes damaging abrasion of the cable, consequently shortening the life of the cable.
Second, because of the two directional change points at the frame bottom lug and the cable guide 12, the cable 13 must extend over a large length from the shift lever to the front derailleur. Obviously, the longer the cable, the greater the elastic elongation thereof when subjected to an operational tension. Therefore, the response accuracy of the front derailleur lowers by that much at the time of operating the shift lever.
Third, since the cable extends round the bottom lug of the bicycle frame and along the down tube, the cable is readily subjected to deposition of mud scattered by the rotating front wheel. Obviously, such mud deposition causes deterioration of the cable due to corrosion or the like.